Lead chromate pigments and process for producing the same



Patented Dec. 13, 1938 LEAD CHROMATE PIGIWENTS AND PROCESS FOR PRODUCING THE. SAME Samuel C. Homing, Newark, N. J assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing.

15 Claims.

This invention relates to lead chromate pigments of improved light fastness, and more particularly to those obtainable by treating an aqueous suspension of lead chromate with an inor- 5 ganic reducing agent, whereby on separation of the pigment from the aqueous slurry, a small amount of the reduction product remains associated with the pigment.

The invention has reference to straight lead chromate pigments, generally termed chrome yellows in the art, as well as to the chrome greens in which the lead chromate is mixed or shaded with iron blues.

The preparation of commercially satisfactory lead chromate pigments requires that the lead chromate be precipitated preferably in the form of very small, microscopic particles. In order to achieve this, it has been customary to add restraining agents, such as acids or their salts, which form insoluble lead salts; tannic acid, tartaric acid or disodium phosphate, to the precipitation medium. These agents prevent the precipitated lead chromate particles from changing to a relatively long, needle-like shape, which is otherwise generally obtained when they are not employed. Chrome greens produced from these long, needle-like lead chromates are generally dull and their shade appears to change when they are viewed from different angles. While such restraining agents function to prevent the precipitated lead chromate particles from growing to relatively long, needle-like shape, the chrome pigments produced are unsatisfactory and commercially deficient, due to their decreased light fastness.

Improvements in light fastness of lead chrome pigments have been attempted by precipitating upon the pigments small amounts of certain insoluble metallic hydroxides, such as those of aluminum, zinc, or tetravalent tin. Neither these agents nor the corresponding salts act as restraining agents against crystal growth of the lead chromate, nor do the corresponding soluble salts have any effect upon light fastness when the same are present in lead chromate pigments.

I have found that the use of the foregoing agents may be dispensed with and that poor light fastness and objectionable particle size growth of lead chromate pigments can be overcome, if the precipitated lead chromate is caused to contact an aqueous solution of a soluble inorganic reducing agent, which possesses the desirable combination of inducing a; restraining action in lead chromate pigment growth without afiecting desired and requisite light fastness.

Application February 20, 1937, Serial No. 126,812

Accordingly, the present invention contemplates the production of a commercially satisfactory lead chromate pigment which is relatively small and minute in particle size and greatly improved in light resistance. These results are obtainable in my invention if the conditions and properties of the reducing agent employed are such that only a relatively slight reduction of the lead chromate takes place.

In accordance with my invention, I contem-='.-

plate, during the production of my improved lead chromate pigment, employing a water-soluble inorganic reducing agent such as a soluble metal sulfide or polysulfide, or soluble stannous ,(bivalent tin) salts. include the soluble metal sulfides or polysulfides of sodium, potassium, calcium, ammonium, or barium. Examples of stannous salts employable herein include the stannous halides such as stannous chloride, stannous bromide, or the sulfates e such as stannous sulfate.

The foregoing water-soluble reducing agents may be utilized in the invention and brought in contact with the lead chromate at practically any time or at any stage of the process of, manufacturing the lead chromate pigment. In most instances, however, it will be found most convenient and therefore preferable to add the agent immediately after the formation of the lead chromate and while maintaining the slurry or suspension containing the lead chromate at a pH of 4.5 to 6.5 during addition.

In adapting my invention to practice, I have found that the amount of soluble inorganic reducing agent requisite for employment may vary; as desired, but that in most instances a relatively small or minor quantity of the reducing agent will usually be sufiicient to obtain the improved benefits of my invention. Generally, however, the amount of agent necessary for employmentz. need not exceed substantially 4% by weight of the lead chromate, and very satisfactory results will obtain if the amount of inorganic reducing agent utilized is within a range of, say, to 3% of the weight of the lead chromate. When.- amounts such as those exemplified are employed, it will be found that only a very slight reduction of the lead chromate will take place by reason of the presence of my reducing agents. In some instances, beneficial results will obtain and: accordingly under such circumstances it will be preferable to employ more than one of my reducing agents for treating the precipitated pigment. For example, the utilization of bothstannous chloride and one or more of the sulphides Examples of the former or polysulphides mentioned will induce beneficial and advantageous results, if it is desired that the reducing agents be severally employed. In instances of several reducing agent employment, it will be found preferable if one agent is initially and independently added and allowed to react prior to addition or employment of the other or remaining agents. Such procedure will obviate the possibility of interaction between the severally employed reducing agents and avoid reduction in their effectiveness as a result. Where employment of several reducing agents is contemplated, optimum results will accrue if the total amount of agents so employed is maintained in relatively minor proportion. If desired, however, the total amount of agents so employed may range as high as 8% with beneficial results obtaining. Preferably, however, optimum results will be found to obtain if the total amount of the several reducing agents utilized approximates substantially 4%, based on the weight of lead chromate. Accordingly, in such instances this latter amount is preferred for use.

In order that the invention may be more clearly understood, the following specific examples are given, each being merely illustrative in character and in no wise in limitation of the broad underlying principles of my invention:-

Example 1 To a suspension of iron blue containing 15 kilograms of pigment was added a solution or slurry containing the equivalent of 111.5 kilograms of lead oxide, 20 kilograms of acetic acid and 14.7 kilograms of nitric acid. To this was then added a solution containing 68.5 kilograms of sodium bichromate. The resulting green slurry was treated with a solution containing 1.5 kilograms of sodium sulfide and then washed by decantation. Finally an additional 0.5 kilogram of sodium sulfide, in solution, was added, followed by the usual filtering and drying to give a chrome green pigment of excellent stability to light.

Example 2 To 165.5 kilograms of lead nitrate in 2000 liters of water at 75 F. was added a solution containing 25 kilograms of soda ash. The pigment was then precipitated by adding, over a period of 30 minutes, 500 liters of a solution containing 58 kilograms of sodium bichromate and 5.5 kilograms of sodium sulfate dissolved together. A solution of 5 kilograms of sodium sulfate was added, followed by a solution of 0.8 kilogram of sodium sulfide. The pigment was washed and a solution of 0.8 kilogram of sodium sulfide was added followed, in turn, by a suspension of iron blue to give the desired shade of green.

Example 3 To 165.5 kilograms of lead nitrate in 2000 liters of water at 75 F. was added a solution containing 25 kilograms of soda ash. The pigment was then precipitated by adding, over a period of 30 minutes, 500 liters of a solution containing 58 kilograms of sodium bichromate and 5.5 kilograms of sodium sulfate dissolved together. A solution of 5 kilograms of sodium sulfate was added followed by a solution of 2 kilograms of ammonium polysulfide. The pigment was washed and then treated with an additional 2 kilograms of ammonium polysulfide in solution followed, in turn, by a suspension of iron blue to give the desired shade of chrome green.

Example 4 To 165.5 kilograms of lead nitrate in 2000 liters of water was added a solution containing 25 kilograms of soda ash. The pigment was then precipitated during the course of about 30 minutes by adding 500 liters of a solution containing 58 kilograms of sodium bichromate and 5.5 kilograms of sodium sulfate dissolved together. A solution of 5 kilograms of sodium sulfate was then added,and followed by a solution of 2 kilograms of stannous chloride. The pigment was washed and a solution of 2 kilograms of stannous chloride was added, followed, in turn, by a suspension of iron blue to give the desired depth of green. After filtering and drying a bright chrome green of excellent light fastness was obtained.

Example 5 To a solution of 165.5 kilograms of lead nitrate in 2000 liters of water was added a solution containing 25 kilograms of soda ash. The pigment was then precipitated by adding, over a period of 30 minutes, 500 liters of a solution containing 58 kilograms of sodium bichromate and 5.5 kilograms of sodium sulfate dissolved together. A solution of 5 kilograms of sodium sulfate was then added followed by a solution of 2 kilograms of stannous sulfate. The pigment was washed by decantation and a solution of 2 kilograms of stannous sulfate was added followed, in turn, by a suspension of iron blue to give the desired shade of green.

Example 6 To a solution of basic lead acetate, containing the equivalent of 111.5 kilograms of lead oxide and 20 kilograms of acetic acid, was added 19.2 kilograms of nitric acid. The volume was adjusted to 6000 liters and the pigment was precipitated by adding 500 liters of a solution containing 55 kilograms of sodium bichromate and 10 kilograms of' sodium sulfate dissolved together. A solution of 2 kilograms of stannous chloride was then added followed by a solution of 5 kilograms of sodium sulfate. The pigment was washed by decantation and a solution of 2 kilograms of stannous chloride was then added followed in turn by a suspension of iron blue to give the desired shade of green.

Example 7 A solution of 165.5 kilograms of lead nitrate in 3500 liters of water was treated with a solution of 11.2 kilograms of soda ash and subsequently with a solution containing 50 kilograms of sodium bichromate and 17 kilograms of sodium sulfate. A, solution of 2 kilograms of stannous chloride was then added, followed by 10 kilograms of soda ash and 5 kilograms of sodium sulfate each dissolved in water. The pigment after washing and drying was a light yellow of excellent light fastness.

Example 8 165.5 kilograms of lead nitrate was dissolved in 2000 liters of water and a solution of 25 kilograms of soda ash was added. The pigment was, then precipitated by adding in the course of thirty minutes to one hour 500 liters of a solution containing 58 kilograms of sodium bichromate and 5.5 kilograms of sodium sulphate dissolved together. 5 kilograms of sodium sulphate was add-, ed followed by a solution containing 2 kilograms of stannous chloride. A solution containing 1.5 kilograms of sodium sulphide was then added. The pigment was washed and shaded with a suspension of iron blue to the desired depth of green was After filtering and drying, a bright chrome green of excellent light fastness was obtained.

The lead chromate pigments obtained as a result of the foregoing examples were of decidedly improved light fastness and of commercially satisfactory, minute, microscopic particle size. In instances where stannous salts were employed, it was found that small amounts of a compound of tin remain associated with the finished pigment.

The startling and unexpected improvement in light fastness and desired particle size which obtains when a lead chromate pigment has been contacted with a solution containing the watersoluble inorganic reducing agents of the character specified is not precisely explainable, but it appears that a surface reduction of the pigment takes place which is sufficient to alter its shade very slightly, but renders the lead chromate pigment highly resistant against light. In addition to the improved light fastness which accrues from my invention, a further advantage exists in the fact that my reducing agents also act to restrain crystal growth of the lead chromate pigment and consequently use of such restraining agents heretofore employed, such as tartaric acid, tannic acid or disodium phosphate, may be dispensed with when the precipitated lead chromate is treated with my inorganic reducing agent.

By the terms relatively slight reducing action, insuflicient to effect substantial reduction, insufiicient to substantially reduce, or insufiicient to effect complete reduction, etc., as employed herein and in the appended claims, I mean to include that limited degree of reduction, presumably on the surface of the pigment particles, which is partial in extent and cannot exceed that degree which would be possible on the basis of the amount of reducing agent employed. As noted above, this amount of reducing agent in no case exceeds 10% of the weight of the lead chromate and preferably ranges from /z% to thereof.

The term sulfide as employed in the appended claims is intended to embrace the soluble mono and polysulfides of sodium, potassium, calcium, ammonium or barium.

I claim as my invention:

1. In a process for producing a pigment comprising lead chromate, the step of adding to a suspension of precipitated lead chromate for reaction therewith a small amount of a reducing agent comprising a water-soluble, inorganic salt from the group consisting of a metal sulfide, stannous halide and stannous sulfate, the amount of agent so employed being sufficient to restrain the crystal growth of the lead chromate.

2. In a process for producing a pigment comprising lead chromate, the step of adding to a substantially neutral aqueous suspension of precipitated lead chromate for reaction therewith .5-8% of a reducing agent comprising a watersoluble, inorganic salt from the group consisting of a metal sulfide, stannous halide and stannous sulfate, the amount of agent so employed being based on the weight of the pigment.

3. A process for producing a lead chromate pigment, comprising adding to precipitated lead chromate for reaction therewith .54% of a reducing agent, comprising a water-soluble, inorganic salt from the group consisting of a metal sulfide, stannous halide and stannous sulfate, the amount of agent so employed being based on the weight of the pigment.

4. In a process for producing a pigment comprising lead chromate, the step of adding to an aqueous suspension of precipitated lead chromate for reaction therewith a small amount of a water-soluble, inorganic stannous salt, the amount of stannous salt so employed being sufficient to restrain the crystal growth of said lead chromate pigment.

5. In a process for producing a pigment comprising lead chromate. the step of adding to an aqueous suspension of precipitated lead chromate for reaction therewith .5-8% of a water-soluble, inorganic stannous salt, the amount of salt so employed being based on the weight of the pigment.

6. In a process for producing a pigment comprising lead chromate, the step of adding to an aqueous suspension of precipitated lead chromate for reaction therewith a small amount of a water-soluble, inorganic sulfide, the amount of sulfide so employed being sufficient to restrain the crystal growth of said lead chromate.

'7. In a process of producing a pigment of improved light fastness which comprises a lead chromate and an iron blue, the steps of producing a suspension of lead chromate and iron blue in a solution, adding to said solution for reaction with said pigment a small amount of a reducing agent, comprising a Water-soluble, inorganic salt from the group consisting of a metal sulfide, stannous halide and stannous sulfate, the amount of reducing agent so employed being suf ficient to restrain the crystal growth of the lead chromate, separating the resultant pigment from the solution and drying the same.

8. The process of claim 4 in which the stannous salt is the chloride.

9. The process of claim 6 in which the sulfide is sodium sulfide.

10. The process of claim 1 in which the reducing agent comprises both stannous chloride and sodium sulfide.

11. The process of claim 7 in which said reducing agent is stannous chloride.

12. The process of claim 7 in which said reducing agent is sodium sulfide.

13. The process of claim '7 in which said reducing agent comprises both sodium sulfide and stannous chloride.

14. A lead chromate pigment of improved light fastness containing a small amount of a reduction product comprising the reaction product of a water-soluble inorganic salt from the group consisting of a. metal sulfide, stannous halide and stannous sulfate with said lead chromate, said reaction product being intimately associated with said pigment during its process of manufacture.

15. A lead chromate pigment of improved light fastness containing a small amount of the reaction product of a water-soluble, inorganic stannous salt with said pigment, said reaction product being intimately associated with said pigment during the manufacture of the latter.

SAMUEL C. HORNING. 

